1. Field of the Invention
The present invention relates to a machine tool that performs ultra-precise machining for which machining accuracy in nano-units is required, and more particularly to a machine tool capable of accurately determining a machining start position.
2. Description of Related Art
In a machine tool that performs ultra-precise machining for which machining accuracy measured in nano-units is required, when performing scribing or the like on a machining surface, in order to improve machining accuracy it is necessary to determine a position of a tool tip point with respect to the workpiece accurately and start machining, and it is also necessary to set a coordinate position of the workpiece machining start point with nano-unit accuracy.
Conventionally, the tool tip position is detected using a sensor or other such special tool detection device or a microscope. As methods of detecting the position of the workpiece, an invention is known that limits an output torque of a rotary actuator and advances the tool toward the workpiece, and in a torque-over state, and moreover when a skip deviation amount exceeds a preset position deviation, skips movement instruction and stops any succeeding movement instruction (see, for example, JP2002-328707A).
As a method that detects the position of the tool with respect to the workpiece, the position of the tool can be detected with the tool detection device used conventionally but the position of the workpiece cannot be ascertained accurately. In addition, with a tool having a sharp tip, the true position of the tip cannot be detected.
Moreover, when detecting using a microscope, a distance between a blade tip and the workpiece is measured and the tool can be brought very close to the workpiece. However, given the limited focal depth of the microscope, as machining progresses sometimes the blade tip can no longer be seen.
In addition, limiting the output torque of the actuator such as a motor that drives the tool and moving the tool toward the workpiece, in a torque-over state obtaining the position when the position deviation exceeds the skip deviation amount, and measuring the workpiece position with respect to the tool as with the invention described in JP2002-328707A, the torque in the torque-over state acts on the tool and the workpiece. In a machine tool that machines ultra-precisely with a machining accuracy measured in nano-units, bearings of movable axes are comprised of fluid bearings such as air bearings, in other words, frictionless bearings. As a result, even a slight force exerted on the movable axes, because the movable axes move, in a torque-over state like that described in JP2002-328707A, when a state is detected in which the position deviation has exceeded a predetermined value, the possibility increases that the workpiece or tool has already moved from a contact position for that workpiece and tool, making it difficult to determine that position with nano-unit accuracy.